Cathodic protection system



- Jan. 28, 1969 J. A. MATHEWS CATHODIC PROTECTION SYSTEM Filed Oct. 22.1965 .N X Z P K: AA/ l g fi WWW/N Q INVENTOR JAMES E. MATHEWS 6 E o 3ATTORNEYS United States Patent 8 Claims The present invention relatesgenerally, as indicated to a protective system and, more particularly,to certain improvements in self-regulating cathodic protection systemsespecially of the type shown and described in US. Letters Patent No.3,143,670, granted to Bernard Husock on Aug. 4, 1964.

In such prior patent, there is disclosed a relatively simple andinexpensive impressed current cathodic protection system which isadapted to provide a substantially constant current at any given anodeeven though there may be a rather wide variance in the resistivity ofthe soil or other electrolyte in which the structure to be protected,such as a gasoline tank, for example, is buried. This substantialllyconstant current is obtained by placing a fixed resistor, which has aresistance at least ten times as great as the resistance between theanode and the protected structure, in series with a rectifier forconverting a typical 115 volt A.C. source to direct current.Accordingly, even should the conductivity of the electrolyte between theanode and protected structure change rather drastically, it has beenfound that the current output will still deviate less than percent.Thus, by employing the protective system of the aforementioned Husockpatent, it is possible to provide the minimum current flow which willprotect the buried structure, since there is no danger of a substantialdrop in such flow as operating conditions change.

However, in some cases it is desirable to provide a cathodic protectionsystem which provides an even more constant current output despitefluctuations in the resistance between the anode and structureprotected, which is a principal object of the present invention.

Another object is to provide an improved cathodic protection system ofthe type indicated which generates much less heat for longer componentlife than the resistor circuit of the aforementioned Husock patent.

Still another object is to provide such cathodic protection system whichmaintains the desired low constant current output at the anodes under alower voltage potential in the vicinity of the protected structure.

Other objects and advantages of the present invention will becomeapparent as the following description proceeds.

To the accomplishment of the foregoing and related ends, the invention,then, comprises the features hereinafter fully described andparticularly pointed out in the claims, the following description andthe annexed drawing setting forth in detail a certain illustrativeembodiment of the invention, this being indicative, however, of but oneof the various ways in which the principles of the invention may beemployed.

In such annexed drawing there is shown a circuit diagram of a preferredform of protection system in accordance with the present invention.

The preferred embodiment of protective system illustrated in the drawingis generally indicated at 1, it being shown schematically in a typicalinstallation providing a substantially constant low current output at ananode 2 buried adjacent but out of contact with a metal gasoline tank 3or the like which is to be protected from corrosion. The current for theprotective system 1 may be supplied from an external 117 volt A.C.source connected to the leads 4 and 5 of the primary winding 6 of a 117v. A.C. primary, 28 v. A.C. secondary, l0 va. step-down transformer 7.

The transformer secondary winding 8 is in turn connected to diagonallyopposite corners 10 and 11 of a bridge rectifier 12 which in the formillustrated is a 1 ampere, v. PRV encapsulated rectifier comprised offour diodes 13 arranged in back-to-back relationship for creating afull-wave direct current output at the remaining two corners 14 and 15.The current from the bridge rectifier 12 always flows in the samedirection from the corner 14 and back to the corner 15. The outputcorner 14 of the rectifier 12 is connected to the emitter 16 of a powertransistor 17 through an emitter bias resistor 18, while the base 19 ofthe transistor is connected through a Zener voltage reference diode 20across the emitter bias resistor 18. In addition, the transistor base 19is connected through a Zener diode reference resistor 21 to groundthrough an electric cable 22 leading to the corner 15 of the rectifier12 and also to the tank 3 which is the cathode. The collector 25 of thetransistor 17 is connected to the anode 2.

The emitter bias resistor 18, which in a typical installation may be a68 ohms, 11 /2 watt resistor, determines the current flow between thebase 19 and emitter 16 of the power transistor 17, thus helping toprovide the correct current value at the anode 2. However, the low valveresistor 18 does not prevent fluctuations of current at the anodes 2, aswhen the load resistance between the anode and tank 3 fluctuates betweenwet and dry periods or for other reasons. Such current fluctuations areprevented by the Zener diode 20 because of its unique operatingcharacteristics which permit it to accommodate changes in output currentwithout a change in voltage, that is, so long as the Zener diodereference resistor 21 has a value which will cause the diode 20 tooperate in its Zener region. In the preferred form shown, the resistor21 is a 4.7K ohms, 2 watt resistor.

From the above discussion, it can now be seen that the transistorizedcurrent regulator 1 of the present invention is basically a groundedbase circuit, with the collector current being essentially equal to theemitter current due to the high gain characteristics of the transistor17. Moreover, the current supplied to the load will remain essentiallyconstant until the load resistance (i.e., the resistance between theanode 2 and tank 3) is increased to a point where the voltage dropacross the emitter bias resistor 18 is as large as the voltage dropacross the transistor base resistor 21.

In actual practice, it has been found that this particular system willprovide a 100 milliamperes nominal current at the anode 2 over a rangeof from 0 to 200 ohms variance in the load resistance, which is a muchgreater variance than is ordinarily found. Thus, by using the protectivesystem of the present invention it is possible to provide the minimumcurrent flow which will protect a buried structure, since there is nodanger of a substantial drop in such flow as operating conditionschange. Furthermore, this system operates at a very low potential at theanodes while still maintaining the desired constant current output,which is advantageous especially where there is a fire hazard as whenthe structure being protected contains a flammable product such asgasoline. Also, because the resistor 18 has a rather low resistance andthe power transistor 17, due to its construction, permits maximumdissipation of heat, the system disclosed herein operates at cooltemperatures for longer component life.

While only one anode and its associated components is shown, it shouldbe understood that a plurality of such anodes may be provided, the exactnumber depending upon the amount of current needed to protect a givenstructure. Thus, if the current output at each anode is 100milliamperes, and 800 milliamperes are required to protect a givenstructure, then it would be necessary to provide eight such anodes withassociated components.

Other modes of applying the principles of the invention may be employed,change being made as regards the details described, provided thefeatures stated in any of the following claims or the equivalent of suchbe employed.

I therefore particularly point out and distinctly claim as my invention:

1. A protective system for objects subject to corrosion comprising atleast one anode adapted to be submerged in an electrolyte in which suchobject is located closely adjacent but out of contact with such object,a transistor comprising an emitter, a base, and a collector, saidcollector being directly connected to said anode, an emitter biasresistor connected to said emitter, said base being con nected through aZener diode across said emitter resistor, and an electric cable adaptedto be connected to such object as the cathode, said base being connectedto ground through a Zener diode reference resistor, said Zener diodealso being connected to ground through said Zener diode referenceresistor.

2. The protective system of claim 1 wherein said emitter resistor has alow resistance and said transistor is a power transistor which permitsmaximum dissipation of heat, whereby said system operates at cooltemperatures for longer component life.

3. The protective system of claim 1 further comprising a step-downtransformer the primary winding of which is adapted to be connected toan A.C. source, a bridge rectifier having input terminals connected tothe secondary winding of said transformer and output terminals connected to said resistors, respectively.

4. The protective system of claim 3 wherein said bridge rectifier iscomprised of four diodes arranged in back-toback relationship forcreating a full-wave direct current output.

5. The protective system of claim 3 wherein said bridge rectifier is a1.0 ampere, 100 v. PRV encapsulated rectifier for converting alternatingcurrent from a 117 v. A.C. source, said transformer is a 117 v. A.C.primary, 28 v. A.C. secondary, va. step-down transformer, said emitterbias resistor is a 68 ohms, 11 /2 watt resistor, and said Zener diodereference resistor is a 4.7K ohms, 2 watt resistor.

6. The protective system of claim 1 wherein there are a plurality ofsaid anodes with associated transistors, emitter bias resistors, Zenerdiodes, and Zener diode reference resistors connected thereto asaforesaid.

7. A protective system for underground metal storage tanks and the likecomprising a plurality of anodes adapted to be buried adjacent but outof contact with such tank, an alternating current source, a plurality oftransistors each including an emitter, a base, and a collector, saidcollector for each transistor being directly connected to one of saidanodes, an emitter bias resistor connected to each of said emitters,said base for each of said transistors being connected through a Zenerdiode across said emitter resistors, and an electric cable adapted to beconnected to such object as the cathode, said base for each resistorbeing connected to ground through a Zener diode reference resistor, saidZener diode also being connected to ground through said Zener diodereference resistors, a step-down transformer having a primary windingwhich is connected to an A.C. source, a bridge rectifier having inputterminals connected to the secondary winding of said transformer andoutput terminals connected to said resistors, respectively.

8. The protective system of claim 7 wherein said bridge rectifier is a1.0 ampere, 100 v. PRV encapsulated rectifier for converting alternatingcurrent from 117 v. A.C. source, said transformer is a 117 v. A.C.primary, 28 v. A.C. secondary, 1O va. step-down transformer, saidemitter bias resistors are 68 ohms, 11 /2 watt resistor, and said Zenerdiode reference resistors are 4.7K ohms, 2 watt resistors.

References Cited UNITED STATES PATENTS 3,143,670 8/1964 Husock 307-3,242,064 3/1966 Byrne 204-196 3,258,612 6/1966 Rubelmann 204196 JOHN H.MACK, Primary Examiner.

T. TUNG, Assistant Examiner.

US. Cl. X.R.

1. A PROTECTIVE SYSTEM FOR OBJECTS SUBJECT TO CORROSION COMPRISING ATLEAST ONE ANODE ADAPTED TO BE SUBMERGED IN AN ELECTROLYTE IN WHICH SUCHOBJECT IS LOCATED CLOSELY ADJACENT BUT OUT OF CONTACT WITH SUCH OBJECT,A TRANSISTOR COMPRISING AN EMITTER, A BASE, AND A COLLECTOR, SAIDCOLLECTOR BEING DIRECTLY CONNECTED TO SAID ANODE, AN EMITTER BIASRESISTOR CONNECTED TO SAID EMITTER, SAID BASE BEING CONNECTED THROUGH AZENER DIODE ACROSS SAID EMITTER RESISTOR, AND AN ELECTRIC CABLE ADAPTEDTO BE CONNECTED TO SUCH OBJECT AS THE CATHODE, SAID BASE BEING CONNECTEDTO GROUND THROUGH A ZENER DIODE REFERENCE RESISTOR, SAID ZENER DIODEALSO BEING CONNECTED TO GROUND THROUGH SAID ZENER DIODE REFERENCERESISTOR.